How to Pass Optional Parameters to a C DLL Function from VB

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, 32-bit only, version 4.0

SUMMARY

It is possible to pass optional parameters to a C DLL function from Visual Basic. Visual Basic supports the Optional Keyword in the Declare statement that tells the compiler to pass a VARIANT of type VT_ERROR if the corresponding parameter is omitted in the function call. Otherwise, if the parameter is not omitted, it is passed according to however it may be declared. In any case, nothing is optional on the C side of things -- the function always accepts the fixed number of parameters for which it has been defined.

This article provides a step-by-step example that demonstrates how to pass optional parameters to a C DLL function.

MORE INFORMATION

Step-by-Step Example

1. Create a 32-bit Windows C DLL with the following function:

         long _stdcall OptionalParamCall(LPSTR pStr, VARIANT op1, VARIANT op2)
         {
            if (op1.vt == VT_ERROR && op1.scode == DISP_E_PARAMNOTFOUND)
            MessageBox (NULL, "Optional Param1 is Empty!", "Test DLL", MB_OK);
   
            if (op2.vt == VT_ERROR && op2.scode == DISP_E_PARAMNOTFOUND)
            MessageBox (NULL, "Optional Param2 is Empty!", "Test DLL", MB_OK);
   
            MessageBox (NULL, pStr, "Test DLL", MB_OK);
            return 1;
         }
   
      Export the function in a .DEF file as follows:
   
         LIBRARY   TESTDLL
   
         CODE      PRELOAD MOVEABLE DISCARDABLE
         DATA      PRELOAD SINGLE
   
         EXPORTS
         OptionalParamCall     @1
   
      Name this DLL as Testdll.dll and put the file into the
      system(Windows 95) or \system32 (Windows NT) directory.
   
   

2. From Visual Basic 4.0 32-bit, open a new project, and add the following code to the general declarations portion of Form1:

      Private Declare Function OptionalParamCall Lib "testdll.dll" _
         (ByVal s As String, Optional ByVal op1, Optional ByVal op2) As Long
   
         Private Sub Form_Click()
            ret& = OptionalParamCall("hello")
            ret& = OptionalParamCall("hello", 7)
            ret& = OptionalParamCall("hello", , "world")
            ret& = OptionalParamCall("hello", 8.2, "Mike")
         End Sub
   
   

3. Run the Visual Basic program, and click the form. A series of Msgboxes will appear to tell you which optional parameters are empty and give the value of the string passed as the first parameter.